Synthesis of chromium N-heterocyclic carbene complexes using chromium Fischer carbenes as a source of chromium carbonyls

Article abstract of DOI:10.1016/j.jorganchem.2007.08.043

Chromium Fischer carbene complexes, [Cr{{double bond, long}OMe(R)}(CO)₅] have been utilized as a source of chromium carbonyls in the synthesis of chromium NHC complexes. Using the synthetic method, chromium complexes of various NHC ligands were isolated in reasonable yields. Moreover, the method can be employed for the synthesis of molybdenum and tungsten NHC compounds.

Full text of DOI:10.1016/j.jorganchem.2007.08.043

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 692 (2007) 5390–5394
www.elsevier.com/locate/jorganchem
Synthesis of chromium N-heterocyclic carbene complexes
using chromium Fischer carbenes as a source of chromium carbonyls
a
a
a
a
Seongjin Kim , Soo Young Choi , Young Tak Lee , Kang Hyun Park ,
b
a,
*
Helmut Sitzmann , Young Keun Chung
a
Intellectual Textile System Research Center, Department of Chemistry, College of Natural Sciences, Seoul National University,
Seoul 151-747, Republic of Korea
b
Fachbereich Chemie, Universiata¨t Kaiserslautern, 67653 Kaiserslautern, Germany
Received 20 July 2007; received in revised form 22 August 2007; accepted 23 August 2007
Available online 14 September 2007
Abstract
Chromium Fischer carbene complexes, [Cr{@OMe(R)}(CO)5] have been utilized as a source of chromium carbonyls in the synthesis
of chromium NHC complexes. Using the synthetic method, chromium complexes of various NHC ligands were isolated in reasonable
yields. Moreover, the method can be employed for the synthesis of molybdenum and tungsten NHC compounds.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: N-heterocyclic carbene; NHC; Chromium complexes; Fischer carbene; Imidazolium salt
1. Introduction
ever, although the first chromium NHC complex was
¨
reported 38 years ago by Ofele [5], the chemistry of chro-
N-heterocyclic carbene (NHC) ligands have recently
become very popular as an alternative to phosphines [1].
Stable NHCs, in particular, display rich coordination
chemistry and have found a multitude of applications as
catalyst components due to their remarkable ligand prop-
erties [2]. Thus, synthetic efforts towards transition metal
complexes containing NHCs have been the subject of
intense interest. Methods for synthesizing these complexes
mainly consist of direct reaction of either the stable Ardu-
engo’s carbene or the imidazolium salt with appropriate
metal fragments, and via four-component condensation
reactions with cyano transition metal complexes [3]. Very
recently, Crabtree et al. reported on the use of imidazoli-
um-2-carboxylates as efficient precursors to NHC com-
plexes of Rh, Ru, Ir, and Pd [4].
mium NHC complexes has not been well developed, pre-
sumably in part due to the lack of generally applicable
synthetic procedures. Thus, studies on the synthesis of
chromium NHC complexes exist in only a handful of
examples. Most of the reported methods [6] used Cr(CO)6,
Cr(CO)5(THF), and Na2Cr2(CO)10 as chromium carbonyl
sources. They reacted directly with NHCs. Sometimes rea-
sonable yields were obtained, but sometimes the yields were
poor. Thus, the NHCs studied were limited to some specific
substrates. Recently, Hahn et al. reported [7a,7b] a tem-
plate synthesis of chromium benzanulated NHC com-
plexes. A similar reaction was also reported by Michelin
¨
et al. [7c] Ofele et al. reported the synthesis of tetrazole
and triazole NHC complexes of group six carbonyls [8].
The development of new methodologies towards prepara-
tion of chromium NHC complexes remains an important
synthetic endeavor.
Amongst Fischer carbenes, the chromium metal com-
pound is one of the most studied metal compounds. How-
Recently, we attempted to synthesize a chromium
complex containing a Fischer carbene and an NHC ligand
by the reaction of [Cr{@OMe(R)}(CO)5] with NHC.
*
Corresponding author. Tel.: +82 2 880 6662; fax: +82 2 889 0310.
E-mail address: ykchung@plaza.snu.ac.kr (Y.K. Chung).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.08.043

S. Kim et al. / Journal of Organometallic Chemistry 692 (2007) 5390–5394
5391
Surprisingly, a chromium NHC complex was obtained as
the sole chromium product. The chromium Fischer carbene
acted as a source of chromium carbonyls. Herein we wish
to communicate our preliminary results for the synthesis
of chromium NHC complexes via a reaction of chromium
Fischer carbenes with NHCs and an X-ray crystal structure
of one of them. Moreover, the method can be employed for
the synthesis of molybdenum and tungsten NHC
complexes.
Fig. 1. Crystal structure of 2A (30% thermal ellipsoid).
2. Results and discussion
We first screened the reaction conditions to find an effi-
cient and general method for the synthesis of chromium
NHC complexes using [Cr{@OMe(R)}(CO)5] (R = Me, n-
Bu, Ph) as a chromium carbonyl source, and also screened
chromium carbonyl sources, such as Cr(CO)6 and
Cr(THF)(CO)5 in order to make a comparison (Table 1).
An NHC ligand, N,N0-bis(2,6-diisopropylphenyl)imi-
dazol-2-ylidene (I-Pr), 1A, was used as a model ligand.
The use of [Cr{@OMe(R)}(CO)5] (R = Me, n-Bu, Ph) as
a chromium carbonyl source in the synthesis of chromium
NHC complexes has never been reported. However, the
nucleophilic substitution of the carbene ligand in chro-
mium Fischer carbenes is already well known [9]. The for-
mation of a chromium NHC complex, 2A, was confirmed
by NMR, IR, elemental analyses, and an X-ray diffraction
study (Fig. 1) [10,11]. The 13C NMR spectrum of 2A
reveals the signal of the carbene C atom at d 199.9, lying
in a typical range of the chromium compounds with unsat-
urated NHCs [6a]. Other spectroscopic data for 2A are in
line with its proposed structure [6]. One notable thing in
the X-ray crystal structure of 2A is that the NHC ligand
is eclipsed to the equatorial chromium tetracarbonyl and
the eclipsed carbonyls are highly bent (\Cr1–C2–O2 =
169.4(2)°) compared to other carbonyls (av. \Cr–C–O =
176.4°) [10]. The yield of the reaction was highly dependent
upon the reaction temperature (entries 1–3 and 4–6),
slightly dependent upon the reaction medium (entries 2
and 5), and was rather insensitive to the chromium Fischer
carbenes (entries 6–8). At room temperature, no reaction
(entries 1 and 4) was observed. However, as the reaction
temperature increased, the yield increased to 62–63%
(entries 6–8) in THF solution. When Cr(CO)6 or
Cr(THF)(CO)5 was used as a source of chromium
carbonyl, poor yields (entries 9–10) were obtained. Thus,
the following reaction conditions were employed to study
other reactions: 1.0 equiv [Cr{@OMe(Ph)}(CO)5], 1.2
equiv NHC, THF, 70 °C (the temperature of the reaction
bath), and for 2 h. Table 2 summarizes the reaction of
[Cr{@OMe(Ph)}(CO)5] with various NHC ligands [11].
Table 2
Synthesis of various chromium NHC complexesa
OMe
N
N
R'
R
N
N
R'
(CO)5Cr
+
R
Ph
Cr(CO)5
X-
Table 1
Screening of chromium sourcea
1
2
X = Cl, Br, I, PF6
N
N
I-Pr
I-Pr
(CO)5Cr(L)
N
N
I-Pr
+
Entry Imidazolium salt (1)
Product (2) Yield (%)b
I-Pr
Cr(CO)5
Cl-
1
2
3
4
R,R0 = 2,6-diisopropylphenyl (I-Pr)
1A
1B
1C
2A 62
2B 67
2C 57
2D 66
2E 50
2F 66
2G 46
2H 34
2I 35
2J 48
2K 29
2C 42
2C 24
R,R0 = mesityl (Mes)
1A
2A
R,R0 = 2,6-dimethylphhenyl
R,R0 = 4-bromo-2,6-dimethylphhenyl 1D
I-Pr = 2,6-diisopropylphenyl
5
6
R = phenyl (Ph), R0 = methyl (Me)
R = Ph, R0 = n-Bu
1E
1F
1G
1H
1I
Entry
Cr complex (L)
Solvent
T (°C)
Yieldb
1
2
3
4
5
6
7
8
9
10
a
Toluene
Toluene
Toluene
THF
THF
THF
THF
THF
THF
THF
r.t.
50
Reflux
r.t.
50
Reflux
Reflux
Reflux
Reflux
Reflux
N.R.
38
53
N.R.
41
62
63
63
Trace
16
7
8
9
10
11c
12d
13d,e
a
R = Ph, R0 = Benzyl (Bn)
R,R0 = Bn
:C(OMe)Ph
R = Bn, R0 = i-Bu
R = n-Bu, R0 = Me
R,R0 = Me
R,R0 = 2,6-dimethylphenyl
R,R0 = 2,6-dimethylphenyl
1J
1K
1C
1C
:C(OMe)Bu
:C(OMe)Me
THF
Reaction conditions: 0.38 mmol imidazolium salt, 10 ml thf,
0.42 mmol KOtBu, 0.32 mmol chromium Fisher carbene, 70 °C, 2 h.
CO
b
Isolated yield.
Reaction temperature: room temperature.
Microwave irradiation, 80 °C, 10 min.
Cr(Co)6 used.
c
Reaction conditions: 0.12 mmol imidazolium salt, 4 ml solvent,
0.13 mmol KOtBu and 0.10 mmol chromium source.
d
b
e
Isolated yield.

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S. Kim et al. / Journal of Organometallic Chemistry 692 (2007) 5390–5394
Under optimized reaction conditions, 2B was obtained in
67% yield from 1B. In the same way, 2C and 2D were
obtained in 57% and 66% yields, respectively. Recently,
microwave radiation has been widely used as a source of
energy [12]. When microwaves are used as a source of
energy, the reaction time can be shortened and, in some
cases, much higher yields than those obtained in the con-
ventional thermal reactions have been achieved [13]. Thus,
we studied a reaction of 1C with [Cr{@OMe(Ph)}(CO)5] or
Cr(CO)6 under irradiation of microwave (at 80 °C for
10 min; entries 12 and 13). After work-up, 2C was obtained
in 42% and 24% yields, respectively. The microwave irradi-
ation was not helpful for the synthesis of 2C. From 1E, 1F,
and 1G compounds 2E, 2F, and 2G were obtained in 50%,
66%, and 46% yields, respectively. When all the substitu-
ents on the NHC were an alkyl group, the yields were
rather poor: compounds 2H–2K were obtained in rather
poor yields, 34%, 35%, 48%, and 29% yields, respectively,
from 1H–1K. The yield of the reaction was dependent upon
the substituent(s) on the NHC: as the number of the aryl
substituent on the NHC increased, the yield increased. This
observation might be related to the stability of the NHC
anion under the reaction conditions [14].
The chromium Fischer carbene was also used as the
chromium carbonyl source in the synthesis of chromium
benzimidazolin-2-ylidene complexes (Table 3) [11].
Although the yields of the reactions were not good enough,
the result demonstrated that the chromium Fischer carbene
was much better than Cr(CO)6 as a source of chromium
carbonyl in the synthesis of chromium NHC complexes.
Employing the same reaction for the synthesis of a chro-
mium triad afforded molybdenum and tungsten com-
pounds (3A and 4A) in 91% and 40% yields, respectively
(Scheme 1). The reason why there is a big difference in
the yields is not clear [15]. When Mo(CO)6 and W(CO)6
were used instead of chromium Fischer carbene, 3A and
4A were obtained in 32% and 21% yields, respectively.
Recently, 3A was prepared in 67% yield by a reaction of
Mo(CO)6 with triethylborane adduct of N-heterocyclic car-
bine [16].
Scheme 1. Synthesis of molbdenum and tungsten NHC complexes.
Scheme 2. Reaction of [(CO)5Cr(PPh3)] and NHC in progression, react
with PPh3.
To synthesize a chromium complex having a phosphine
and an NHC ligand, triphenyl-phospine (pentacarbon-
yl)chromium was treated with an NHC ligand. Interest-
ingly, an NHC-substituted chromium carbonyl compound
was obtained in 44% yield (Scheme 2). Thus, instead of car-
bonyl, triphenylphosphine was replaced by the NHC. A
similar substitution reaction was reported for a reaction
of VðCOÞ5PRꢀ3 ðR ¼ Ph; n ꢀ BuÞ with P(OPh)3 [17]. Thus,
in order to make a chromium complex having a phosphine
and an NHC ligand, the reaction route changed to react
(NHC)Cr(CO)5 with PPh3. However, the formation of a
chromium complex bearing NHC and phosphine was not
observed.
3. Conclusions
In conclusion, we have developed a new synthetic
method for producing chromium complexes with NHC
ligands via a reaction of chromium Fischer carbenes with
NHC ligands. We found a correlation between the yield
of the reaction and the substituent(s) on the NHC for a ser-
ies of chromium NHC complexes. We expect that our syn-
thetic study will boost the study of the chemistry of
chromium NHC complexes in the near future.
4. Experimental
Table 3
General. Thin-layer chromatography (TLC) was per-
formed on 0.25 mm Merck silica gel coated plates treated
with a UV (254 nm). TLC plates were visualized by ultra-
violet light. THF was purified by distillation from
sodium/benzophenone under nitrogen and toluene was dis-
tilled from sodium under nitrogen. Reagents were pur-
chased from Aldrich Chemical Co. and Strem Chemical
Co. and were used as received. All yields are based upon
isolated materials. 1H and 13C NMR spectra were obtained
with a Bruker 300 MHz spectrometer. Coupling constants
(J values) are reported in hertz (Hz), and spin multiplicities
are indicated by the following symbols: s (singlet), d
(doublet), t (triplet), q (quartet), and m (multiplet). Melting
points were obtained with a MEL-TEMP II (Laboratory
Synthesis of chromium benzimidazolin-2-ylidene complexes using chro-
mium Fischer carbenea
OMe
(CO)5Cr
+
N
N
R
Ph
R
N
N
R
R
X-
Cr(CO)
1
2
X = Cl, Br, I
2L (R = Me)
42
16
2M (R = n-Bu)
44
Trace
2N (R = Bn)
42
N.R.
Yield (%)
Yield (%)b
a
Reaction conditions: 0.38 mmol imidazolium salt, 10 ml thf,
0.42 mmol KOtBu 0.32 mmol chromium Fisher carbene, 70 °C, 2 h.
b
Cr(Co)6 used.

S. Kim et al. / Journal of Organometallic Chemistry 692 (2007) 5390–5394
5393
Devices). Infrared spectra were recorded on a Shimadzu
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Seoul National University. High-resolution mass spectra
were obtained at Korea Basic Science Institute (Daegu,
Korea). High-resolution mass spectral (HRMS) data are
reported in the form of m/z (intensity relative to base
peak = 100). Flash chromatography was performed using
Merck Silica Gel 60 (230–400 mesh).
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˚
˚
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3
˚
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[11] Supporting Information Available: Detailed experimental procedures,
new characterization data, 1H and 13C NMR spectra of all
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´
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(MOEHRD) (KRF-2005-070-C00072) and the SRC/ERC
Program of MOST/KOSEF (R11-2005-065). Y.T.L.
thanks to the BK21 fellowship and S.Y.C. thanks to the
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graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Cen-
tre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the
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S. Kim et al. / Journal of Organometallic Chemistry 692 (2007) 5390–5394
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